1. Field of the Invention
The present invention relates to a blackbody furnace, and particularly to a blackbody furnace for lowering its temperature fast.
2. Description of Related Art
A blackbody furnace is an important apparatus in the industry for calibrating an infrared thermometer and thermal image instrument. The furnace comprises a radiation cavity, heater, thermally insulating layer, housing, temperature controller, airflow controller and high-pressure air source. The radiation cavity is a radiation source of the blackbody furnace. The radiative intensity and wavelength of the radiation source is relevant to the temperature by itself, and the relation could be described by Planck""s law. The heater is mainly used to heat the radiation cavity. The thermally insulating layer is used to reduce heat dissipation from the heater to the housing, thereby reducing heat loss, enhancing thermal efficiency and reducing power consumption. From the viewpoint of the temperature increase of the blackbody furnace, a thermally insulating layer having a good heat isolation is a better choice. However, from the viewpoint of the temperature decrease of the blackbody furnace, a thermally insulating layer having a good heat isolation will cause the temperature of the furnace to lower at a very low speed, especially when the temperature of the blackbody furnace is near the room temperature. Therefore, the prior blackbody furnace has a common characteristic that the speed to lower the temperature (when the temperature is under 150xc2x0 C.) is low. It is well known that the optimal control parameters (proportion, integration, differential) of the temperature controller are different at a high temperature region, middle temperature region and low temperature region. Therefore a temperature overshooting often occurs. If a temperature overshooting occurs at a low temperature region, or the temperature setting is changed to a value lower than furnace temperature, a user always needs to wait for a long time until the blackbody furnace reaches a desired temperature.
Accordingly, an object of the present invention is to propose a blackbody furnace for lowering its temperature fast. Besides, the speed at which the temperature of the furnace of the present invention increase is faster than that of a prior blackbody furnace.
For raising the stability to control the temperature, the material of the radiation cavity of the blackbody furnace is commonly selected from materials of large enthalpy or a large thermal mass. Therefore, the radiation cavity is the component having the largest thermal mass in the blackbody furnace, and is placed in the center of the blackbody furnace. Since a thermally insulating layer is used to surround the radiation cavity, in the process of lowering the temperature, the heat induced in the radiation cavity is difficult to dissipate from side walls of the blackbody furnace. In other words, most heat will dissipate from an opening at the front side by radiation. According to Stefan Boltzmann Law, the total energy of heat radiation is proportional to an absolute temperature to the power of four. The more the temperature is, the stronger the radiation energy is. Therefore, the dissipation from the opening of the blackbody furnace is little at the low temperature region, and that is why the prior blackbody furnace has a low speed to lower the temperature at the low temperature region.
Ways to transfer heat has heat conduction, heat convection and heat radiation. As mentioned above, the effect of heat conduction is poor due to the thermally insulating layer, and the heat radiation is also poor at the low temperature region. Therefore, the optimal way to dissipate heat is heat convection. There is an air channel at the bottom of the radiation cavity of the blackbody furnace of the present invention to conduct the interior of the radiation cavity with the exterior. The air channel is connected to a high-pressure air source through an airflow controller. In the process of lowering the temperature of the blackbody furnace, an airflow controller will be actuated by a temperature controller or operated by an operator, and a high-pressure air will enter the radiation cavity through the air chamber for reducing the temperature of the radiation cavity. Finally, the hot air will be vented from the opening. The present invention enhances the efficiency of the heat dissipation by forced convection to achieve the purpose of fast lowering the temperature of the blackbody furnace.
A further object of the present invention is to save power. The heat-isolation effect of the thermally insulating layer will not affect the speed of lowering the temperature, and the heat loss of the present invention is less than that of prior arts in the process of heating. Therefore, the speed at which the temperature increases will be faster than that of prior arts.
A still further object of the present invention is to improve the temperature uniformity of the radiation cavity of the blackbody furnace. Although the heat isolation effect of the thermally insulating layer will not obviously affect the speed of lowering the temperature, the temperature gradient between the middle section and one of two ends of the radiation cavity in the blackbody furnace will be reduced, thereby improving the temperature uniformity of the radiation cavity of the blackbody furnace.
The opening of the air channel of the blackbody furnace of the present invention is situated at the intersection of a side wall of the radiation cavity and the bottom of the radiation cavity, the change on emissivity can be neglected.